Spatial distribution oftwomaternal messengers inParacentrotus lividus during oogenesis andembryogenesis

Wedemonstrated that twomRNAsthat are synthesized during thevitellogenic period ofoogenesis andthat codeforcell surface proteins areasymmetrically distributed in theunfertilized eggofParacentrotus lividus. Atfertilization, these RNAsrapidly localize inthecortical zoneattheanimal pole oftheegg. Theyarethendetected inthemesomeres and themacromeres, butnotinthemicromeres, andthereafter are found intheectoderm butnotinthevegetal plate, mesenchyme cells, orearly intestine. Theydisappear inlate gastrula. The proteins synthesized bythese mRNAsshowthesameterritorial location during theperiod examined here, which included the unfertilized eggandthe16-blastomere stage. These conclusions werereached onthebasis ofinsitu hybridization andimmu- ntaningexperiments, aswell asNorthern andWestern blot analyses ofisolated blastomeres. Thepossible sinificance of this asymmetric distribution ofthese twomRNAsandproteins intheestablishment oftheanimal/vegetal axis isdiscussed. Determination oftheembryonic axeshaslong beenacentral problem indevelopmental biology. Recent studies inDro- sophila ontheunderlying molecular mechanisms havedem- onstrated that information that specifies theanteroposterior andthedorsoventral axesoftheeggisalready laid down during oogenesis (1) andthat acascade ofevents determines theposition oftheheadandofthethorax. Certain maternal genes lead tolocalization oftheproduct ofthebicoid gene, atranscription factor, producing ananterior/posterior con- centration gradient that regulates zygotic target genes (2, 3). A maternal RNA,nanos, theproduct ofwhich prevents the translation ofatranscription factor encoded byaubiquitous maternal RNAisinitially localized attheposterior pole (4). Another cascade ofevents occurs intheestablishment ofthe dorsoventral axis inwhich theproduct ofthematernal Toll gene, probably amembrane receptor, andthematernal dorsal geneareinvolved (5, 6). Thepossibility that determination of theembryonic axesiscorrelated withdifferent locations of maternal RNAsindifferent eggterritories hasalsobeen suggested fortheeggs ofXenopus, where Melton (7, 8)found that theVglRNA,whoseproduct shares homologies with a type(3transforming growth factor, istransferred tothe vegetal poleoftheoocyte atthetimeofits maturation. Incontrast toDrosophila andXenopus, nothing isknown todate about themolecular basis ofaxis determination insea urchin embryos, wheretheproblem ofanunderlying mech- anismwasfirst raised atthebeginning ofthis century. Microsurgery experiments performed mostly byHorstadius (9)haddemonstrated that ifunfertilized eggswereequato- rially cutinto twohalves andeachhalf wasfertilized, the animal halves always developed into ciliated hollow spheres ("animalized" embryos), whereas thevegetal halves pro- duced"vegetalized" embryos withanexcess ofvegetal structures; while observation indicated thepresence ofsome asymmetry along theanimal/vegetal axis oftheunfertilized seaurchin egg, nosatisfactory explanation forthis asymme- trywasprovided, except forthesuggestion ofadouble gradient proposed in1928byRunnstrom (10, 11). There was noevidence ofadifferential RNAlocalization inechinoderm eggs, ashasbeendemonstrated inXenopus andDrosophila eggs. Wereport herethat twomaternal mRNAs,bepi andbep4, bothof1.4kb,areasymmetrically distributed intheunfer- tilized eggofParacentrotus lividus. Theybelong toamulti- genefamily andcodeforproteins that become localized inthe cell surface during early embryogenesis andmayberelevant forcell adhesion (12, 13). Their molecular massis33kDa, and their messengers showaparticular structural organization; twowell-conserved domains surrounding asingle specific domain (12, 13). Wereport here that both these mRNAsand thecorresponding proteins arelocated exclusively inone-half oftheunfertilized egg(probably theanimal half) andbecome localized attheanimal poleimmediately after fertilization. Those parts oftheembryo that arederived fromthevegetal half oftheeggaredevoid ofthese twomRNAs.After the gastrula stage, these mRNAsarenolonger foundinthe embryo.